When the U.S. Senate takes up the issue of “therapeutic cloning” in the coming weeks, it will be considering a novel technique aimed at obtaining embryonic stem cells, which could potentially be used to treat such diseases as Alzheimer’s disease, Parkinson’s disease, cardiovascular disease, diabetes and spinal cord injury.
Therapeutic cloning is one of several possible strategies scientists have identified for obtaining embryonic stem cells.
UCSF has played a significant role in the effort to obtain embryonic stem cells for study. Last year, researchers in the laboratory of Roger Pedersen, PhD, at the time UCSF professor and research director, reproductive genetics unit, department of obstetrics, gynecology and reproductive sciences, isolated two lines of embryonic stem cells, which have been included on the National Institutes of Health Stem Cell Registry http://escr.nih.gov/.
These cell lines were obtained from embryos donated with informed consent from women undergoing fertility treatment in in vitro fertilization (IVF) clinics. UCSF is one of only two academic institutions in the United States that produced cells lines that qualified for inclusion on the Stem Cell Registry, established by President George Bush last August.
UCSF will begin making these cell lines available to academic researchers for study in the coming months.
UCSF researchers are now working to obtain additional human embryonic stem cell lines from embryos donated from patients in the IVF clinic. Many high quality cell lines will be needed in order to determine the true potential of stem cells in the treatment of diseases, they say. In addition, all of the registry cell lines - including those derived at UCSF—were exposed to mouse cells and bovine serum for their growth and maintenance, which could disqualify them for transplantation into patients. The UCSF researchers’ goals are to develop additional high-quality cell lines that could be used for basic research studies, and to develop methods for deriving and maintaining stem cells without exposure to the mouse and bovine factors.
At times during the past three years, UCSF researchers have also attempted to obtain stem cells using somatic cell nuclear transfer, colloquially known as “therapeutic cloning.” Somatic cell nuclear transfer is considered a possible approach for obtaining embryonic stem cells that would be immunologically compatible with individual patients who could benefit from cell transplant therapy to treat their diseases. The researchers say that studies of the technique could also lead to other therapeutic strategies for treating disease.
In somatic cell nuclear transfer, the DNA of an individual is transferred to an egg from which the DNA has been removed. This process leads to the development of a single-celled embryo in the petri dish that genetically matches the person who donated the DNA. The goal of conducting somatic cell nuclear transfer would be to induce the embryo to proliferate for five to seven days, until it was a microscopic hollow ball made up of 100 to 150 cells. At this stage, the embryo is a primitive structure known as a blastocyst, and it is at this point that embryonic stem cells develop. The objective would be to obtain embryonic stem cells from the blastocyst.
To date, no evidence has been published in a recognized scientific journal that scientists have succeeded in maintaining human embryos derived by somatic cell nuclear transfer to the blastocyst stage. Thus, no scientists have reported success in obtaining embryonic stem cells from embryos developed by somatic cell nuclear transfer.
While the UCSF scientists do not regard their own findings as sufficient for publication in a peer-reviewed journal, they say they have begun to gain insights that could prove helpful in the future.
UCSF is not currently conducting somatic cell nuclear transfer studies; however, research protocols are in place that would permit scientists to resume the studies, says Keith Yamamoto, PhD, UCSF School of Medicine vice dean for research.
“The field of human embryonic stem cell research is in its infancy, and will require years of study in laboratories throughout the world,” says Yamamoto. “However, the potential to add to our basic knowledge of human development and to establish therapeutic applications is enormous. It is critical that scientists be given the opportunity to carry out a broad-based, deep examination of multiple experimental strategies, particularly at this early stage in the evolution of the field.”
It is equally critical, Yamamoto says, that somatic cell nuclear transfer studies be conducted in the context of a thoughtful, careful review of the ethical issues pertaining to the research.
As is the case with all UCSF research protocols involving human biological material, the studies have been reviewed by the University’s Committee on Human Research. In addition, they have been reviewed by a UCSF bioethicist, and the Dean’s Executive Committee of the School of Medicine. They also were stringently peer reviewed by UC scientists for BioSTAR, the University of California matching-grants program that forges partnerships among University of California scientists and businesses.
“There are several ways in which studying somatic cell nuclear transfer should prove uniquely valuable for developing human therapies,” says Pedersen, who led the UCSF somatic cell nuclear transfer studies. Pedersen is now professor emeritus at UCSF and a faculty member of the University of Cambridge. “An obvious benefit would be obtaining embryonic stem cells that were immunologically compatible with individual patients.”
In addition, he says, studying somatic cell nuclear transfer could lead to an alternative method of obtaining embryonic stem cells, one that involved adult cells. “Studying somatic cell nuclear transfer is the only way to determine whether human eggs can revert adult DNA to its embryonic state so that it can specialize into all tissues of the body,” he says. This rejuvenating process, known as “reprogramming,” has been observed in animal studies when DNA of adult cells has been placed in the egg by somatic cell nuclear transfer. “If scientists were armed with a fundamental understanding of reprogramming, they might ultimately be able to reprogram a patient’s own adult cells, such as skin cells, to revert to the embryonic state, thereby providing another source of embryonic stem cells,” Pedersen says.
“Studies on somatic cell nuclear transfer could also give us insight into human development that could lead to an understanding of birth defects and infertility,” he says.
UCSF researchers conducted their somatic cell nuclear transfer studies using eggs that had failed to fertilize in the fertility clinic and otherwise would have been discarded. These eggs were donated with informed consent by women participating in fertility treatments. The DNA that was transferred into the eggs came from human ovarian cells donated by women after they had undergone in vitro fertilization procedures and, previously, from other cell types.
The studies were conducted with funding from Geron Corp., a biotechnology company in Menlo Park, California, with matching grants from BioSTAR, the University of California matching-grants program that forges partnerships among University of California scientists and businesses. The experiments were carefully separated from any studies that involved federally funded staff or resources.
In view of the pending legislation regarding somatic cell nuclear transfer, several pre-eminent institutions or groups recently issued statements of support for further studies of somatic cell nuclear transfer. These include the National Academy of Sciences, the American Association for the Advancement of Science, a group of 40 Nobel Laureates, an advisory panel appointed by the state of California and a special committee of the House of Lords in England.
UCSF has a long tradition in the field of embryonic stem cell research. UCSF developmental biologist Gail Martin, PhD, co-discovered embryonic stem cells in mouse studies in the early 1980s, and coined the term. Developmental biologist Didier Stainier, PhD, is pioneering studies of embryonic stem cells in zebrafish, identifying genes that contribute to the development of heart stem cells. And neurologist Arturo Alvarez-Buylla, PhD, recently discovered the origin of the human brain’s stem cells, and is exploring whether it is these cells that, when cancerous, lead to brain tumors.
“UCSF is pleased to be contributing to the early exploration of the embryonic stem cell field. Our hope is that new and novel therapies will eventually emerge through the collaboration of researchers at public institutions, in cooperation with private institutions, industry and international agencies. It will take active imaginations and a comprehensive exploration to determine the potential of the stem cell field, which could offer hope for patients with currently untreatable diseases. The best chance for achieving success is to engage and fuel the public research enterprise.” says Yamamoto.
Background on somatic cell nuclear transfer, or “therapeutic cloning”:
During recent years, the term “therapeutic cloning” has emerged in the popular press as a way of describing a research strategy that could be used to derive embryonic stem cells that would be immunologically compatible with individual patients needing cell transplant therapy.
The scientific term for the strategy is “somatic cell nuclear transfer,” and it would be the initial step used either to derive embryonic stem cells that could be used to treat patients (therapeutic cloning) or, potentially, to create cloned people (reproductive cloning.)
Therapeutic cloning and reproductive cloning only share an initial step - that which involves transferring the DNA from the cell of an individual to an egg from which the DNA has been removed.
This process leads to the development of an embryo in the petri dish that genetically matches the person who donated the DNA. In therapeutic cloning, an embryo would be grown only to the stage where it was made up of 100 to 150 cells - a microscopic hollow ball of cells the size of a pin point - and it would remain in a petri dish in the lab. At this stage, the embryo, known as a blastocyst, contains no distinctive body tissues, and would never be transplanted into a woman’s womb. The stem cells would be obtained from this mass.
Scientists are interested in carrying out somatic cell nuclear transfer because it could provide a way to obtain stem cells that were genetically identical to the donors contributing the DNA.
Embryonic stem cells have the potential to differentiate into each of the 200 tissue types of the body. If scientists were successful in deriving human embryonic stem cells from blastocysts generated by somatic cell nuclear transfer, they would attempt to prompt them to differentiate into specialized cells, such as heart, neural or pancreatic cells, that could be transplanted into patients to treat varying disorders. In theory, a patient could donate his or her DNA for a somatic cell nuclear transfer procedure, resulting ultimately in stem cells that would be immunologically compatible with tissue for his or her therapeutic use.
Studies of somatic cell nuclear transfer with human cells could also lead to a fundamental understanding of reprogramming, or the process by which the DNA of an adult cell can acquire the ability to specialize into all body tissues. So far, studies of reprogramming have only been done in laboratory animals and some livestock. Scientists believe that by ferreting out the basis of reprogramming in eggs, through somatic cell nuclear transfer studies, they may be able to confer this ability on a patient’s own adult cells, without using eggs. In other words, if scientists were armed with a fundamental understanding of reprogramming, they might ultimately be able to reprogram a patient’s own cell, such as a skin cell, to revert to the embryonic state, thereby providing another source of embryonic stem cells.
In March, researchers at the Whitehead Institute for Biomedical Research reported, for the first time, a “proof of principle” that somatic cell nuclear transfer therapy could prove possible. In their study, mouse embryonic stem cells were used to generate blood stem cells that were then transplanted into mice that had an immunodeficiency disease, curing the genetic defect that resulted in their immunodeficiency. Several other recent studies have also revealed the ability of mouse embryonic stem cells derived by somatic cell nuclear transfer therapy to form a wide variety of tissues.
Also in March, researchers in Florida and Great Britain reported that adult stem cells, often cited as a possible therapeutic alternative to embryonic stem cells, might not have the potential scientists have hoped.
Their studies in mice indicated that adult stem cells transplanted into mouse brains did not differentiate into specialized cell types, as some previous studies have suggested. Rather, the cells simply fused with existing specialized cells, thus only appearing to have evolved into the specialized cells that would potentially have served to replenish damaged tissue.